Reactive Transport Modeling of Natural Gas Molecular and Isotopic Evolution During Diffusive Transport in the Subsurface

Reactive transport modeling was employed to investigate the relative importance of fractionations associated with gas solubility, sorption and diffusive transport on dissolved methane, ethane and propane concentrations and the isotopic composition of carbon in methane (delta C-13(1)) in groundwater. Temperature, pressure and salinity dependencies for the hydrocarbon gases were incorporated. Gas molecular ratios, C-1/(C-2 + C-3), increased with diffusive transport, transitioning from thermogenic values to values typically indicative of biogenic gas sources, >1,000, at the leading edge of the diffusive front. Diffusive isotopic fractionation had a large effect on delta C-13(1) values, with fractionations ranging from -36 parts per thousand to -107 parts per thousand, the difference being a function of the diffusive fractionation factor (alpha(D0)). Larger fractionations resulted from alpha(D0) determined at relatively low pressures, 5-50 atm, and temperatures, 20 degrees C-25 degrees C. Less fractionation occurred with alpha(D0) measured at higher pressures and temperature, 30-89 atm and 90 degrees C. The extremely depleted delta C-13(1) values indicated from the modeling, less than -110 parts per thousand, have not been observed in shallow groundwater, suggesting that diffusive fractionation of delta C-13(1) is offset by other processes such as microbial oxidation. 2(k) factorial analysis was used to assess the model sensitivity to specific parameters: estimations of hydrocarbon travel distance are most sensitive to porosity and tortuosity, while the molecular ratio was most sensitive to the free-water diffusion coefficient, and the isotopic fractionation was most sensitive to alpha(D0). The magnitude of diffusive fractionation on the molecular and isotopic composition of transported hydrocarbon gas may be similar to fractionations from microbial oxidation and mixing between different sources.

Automated summary

Reactive transport modeling was used to study the influence of gas solubility, sorption, and diffusive transport on dissolved methane, ethane, propane concentrations, and isotopic composition in groundwater. Diffusive isotopic fractionation had a significant impact on delta C-13(1) values, resulting in extremely depleted values. The modeling also showed that the diffusive fractionation of delta C-13(1) is offset by other processes like microbial oxidation.